A light source device according to an embodiment includes a solid-state light source, and an optical transmission fiber. The optical transmission fiber includes a wavelength conversion core, a light guide core, and a clad, the wavelength conversion core absorbing excitation light to generate a population inversion state of electrons and including a wavelength conversion material emitting wavelength converted light in a visible light region, the light guide core covering a periphery of the wavelength conversion core and transmitting the wavelength converted light from a first end face side to a second end face side, and the clad covering a periphery of the light guide core. In the optical transmission fiber, the wavelength converted light propagating in the light guide core generates stimulated emission, and the excitation light and the wavelength converted light amplified by the stimulated emission are emitted from the second end face.